When the alignment of an electron microscope is performed, either the voltage center or the current center is brought into the middle of the viewing screen. Generally, when the accelerating voltage is changed, the image wobbles about a point. Accordingly, if the accelerating voltage varies, the effect of this variation on the final image is minimized by adjusting the incident direction of the electron beam impinging on the specimen in such a way that this point is brought into the middle of the final image. This alignment is herein referred to as a voltage center alignment. When the excitation current supplied to the objective lens is varied, the image wobbles about a point. The incident direction of the electron beam falling on the specimen is adjusted so that this point is located at the center of the final image. This is herein referred to as a current center alignment. Once the current center alignment is complete, if the objective-lens current is varied slightly, the center of the image does not move.
A beam wobbler is incorporated in an electron microscope to facilitate these alignment operations. This wobbler modulates the accelerating voltage or the objective-lens current with a sinusoidal wave, triangular wave, saw-toothed wave, or other wave. When the wobbler is in operation, the still, or motionless, center of the image can be clearly discerned. That is, the position of the voltage center or the current center can be known clearly. This center is brought into the center of the final image. In this way, the voltage center alignment or current center alignment is performed.
However, the coaxial coma that is one kind of aberration remains even if the above-described alignments are made. Hence, it is impossible to sufficiently increase the resolution of the electron microscope. Specifically, if such axial coma is large, the quality of the final image differs from location to location. For example, if the aberrations are corrected so as to eliminate the astigmatism in the center of the image, the image quality at the edge of the micrograph is lower, which is an impediment to correct interpretation of the image.
In analytical electron microscopes developed recently, the magnetic field produced in front of the objective lens is made strong. Therefore, the presence of the axial coma presents more serious problems. An alignment method of removing the axial coma is described by F. Zemlin et al. in Ultramicroscopy 3 (1978), pp. 49-60. In this method, the direction a in which no axial coma exists is found in the manner described below. The electron beam that is caused to enter a specimen S is tilted about this direction a by the same angle in positive and negative directions b.sup.+ and b.sup.- as shown in FIG. 1. Similarly, the beam is made to hit the specimen while the beam is tilted in directions c.sup.+ and c.sup.-. The microscopist adjusts the tilt of the beam until the two images look exactly the same. In this way, the direction a is found.
In order to find the direction in which no axial coma is present, skillful electron micrograph observation techniques are required. Also, the use of an image processing apparatus is recommended in addition to the body of the microscope. Furthermore, an amorphous thin film is needed as a specimen. Consequently, it cannot be said that these operations can be performed routinely. Therefore, the present situation is that no countermeasure has been taken against the reduction in the resolution due to the axial coma.